My guess is that Subaru allows no provision for a 7-pin hitch as this is generally for heavier towing applications than the Outback is rated for. AFAIK they have provided for 4-pin only.I have a new 2013 Outback 2.5i that I am modifying to tow a popup camper.
In the owners manual, page 12-10 "Fuse panel located in the passenger compartment" identifies circuit #1 as "Trailer hitch connector". Does anyone know if this is intended to supply power to the trailer (as opposed to simply protecting a lighting circuit)? If so, where is the wire for this circuit? I've already attached a T-connector to the trailer wiring connector located on the left in the cargo area, but this only gives me 4-pin lighting output. I didn't see any other wire in this area that might be the 15A power wire that I'd attach to a 7-pin connector.
If you are looking to add 7-pin you will probably need to find power sources on your own. I would use the OEM connections for turn signals, parking, and brake lights. For your back up lights you can just tie into one side of the taillights. For a trailer brake circuit I would wire this directly to the battery or to the fusebox with an inline fuse, your trailer brake kit probably has a fuse and instructions. For a battery charger circuit I would wire it into the fuse box but I believe it is recommended that this circuit is on at least a 30A relay and fused circuit.